The role of the pH in the anaerobic digestion of domestic primary sludge at mesophilic temperature was evaluated. Since solubilization of organic materials is seriously affected by pH, much research has been performed for the evaluation of significant pH influence on the production of soluble organic carbon in the acid phase. In this study, the performance of continuously stirred anaerobic reactors for the production of soluble chemical oxygen demand (SCOD) and volatile fatty acids (VFAs) in domestic primary sludge was investigated. In the first anaerobic reactor, the pH was not controlled (pH-uncontrolled) whereas the pH was fixed at 6.5 (pH-controlled) in the second reactor. Two sets were performed. Results of both sets indicated that, VSS was removed with a corresponding production of VFAs and SCOD in both reactors. However, production of VFAs and SCOD stopped earlier in the pH-controlled reactors. Methanogenic phase started to operate at earlier times whereas hydrolysis and fermentation were still operating in the pH-uncontrolled reactors. The process stability and efficiency of anaerobic digestion of substrates such as domestic primary sludge can be improved by anaerobic digestion elutriated phased treatment (ADEPT) in which the acid elutriation reactor and methanogenic reactor are separated. In this respect, ADEPT was introduced.